Semiconductor lasers, such as edge emitting lasers and vertical cavity surface emitting lasers (VCSELs) are well known, and are formed in a wide variety of configurations. Edge emitting lasers typically consist of an intrinsic junction region formed between two layers of semiconductor material each having been treated with a different type of impurity. When a large electrical current is passed through such a device, laser light emerges from the cleaved facets in the plane of the junction region. VCSELs, on the other hand, typically include an active area disposed or sandwiched between two mirror stacks on a semiconductor substrate. The laser is activated by driving an electrical current through the two mirror stacks and the active area. This is generally accomplished by placing a first electrode across the mirror stack at one end of the laser and a second electrode across the other mirror stack at the other end of the laser. One of the electrodes generally defines a central opening therethrough for the emission of light.
In operation, a threshold level of current must be forced through the active region of the semiconductor laser for lasing to occur. The threshold level is reached when the gain for the stimulated emissions in the cavity equals to or exceeds the loss in the cavity. Upon reaching threshold, the light output rises rapidly with the current, with most of the current resulting in laser emissions.
Edge emitting semiconductor lasers are very sensitive to temperature in output power and threshold current. Most applications involving the use of semiconductor lasers require that the laser output power be stabilized. For example, in the optical data storage application, changes in light intensity of a received signal correspond to the data bits being read. Thus, it is important that the power of the source signal be maintained at a fixed or constant value so that the power of the received data bits corresponds to the actual data rather than the drift of the source signal due to environmental changes.
The power of the source signal may be maintained at a fixed or constant value through Automatic Power Control (APC). APC of semiconductor lasers allows for a constant and consistent output from the lasers. Generally, APC of edge emitting laser devices is easily achieved because edge emitting devices emit light from two ends. APC of edge emitting lasers may be achieved by using one of the light emitting ends to measure the optical power output, which is subsequently used to adjust the electrical power input to the edge emitting device and, thereby, adjusting the optical power output. Typically this is achieved by placing a photosensor behind the laser to receive the backward emission. The power of this backward emission is proportional to the power of the forward emission. Adjustment of the power is achieved by establishing a feedback loop to adjust the laser injection current to maintain a constant forward laser emission power.
While this type of APC works with a single laser, an array of lasers spaced closely apart will experience problems in maintaining the constant power operation for each individual laser due to the overlap in laser emission from the array. More particularly, monitoring of individual lasers which compose the array is performed simultaneously using an array of photosensor, resulting in an overlap in laser emission from the array. For example, a one-dimensional array of four lasers spaced at a maximum of thirty microns apart, will require an array of four photosensors to receive the backward emission from the four lasers. Due to the overlap of the four beams, each photosensor will receive the laser emission from its own corresponding laser, as well as from its neighboring lasers. It is therefore not feasible to use the photocurrent variation of the photosensors to control the corresponding laser output power.
In applications such as parallel optical data storage and parallel laser printing it is necessary or highly desirable to maintain a fixed output power from each laser which composes the array. One solution to the array power monitoring problem is to utilize a single large photosensor to receive the backward laser emission from all four lasers in the array. APC of each individual laser is achieved by controlling the monitor timing during the current sweeping across the four lasers. This technique, while feasible to achieve APC of each individual laser, is difficult to incorporate into the packaging of laser arrays, especially when large laser arrays are used.
Thus, it can be readily seen that conventional APC of laser devices has several disadvantages and problems, thus not easily enabling their manufacture in low cost volume applications and compact packaging. Accordingly, it would be advantageous to have a method and apparatus for controlling the power of semiconductor lasers, particularly edge emitting laser devices, that are reliable, cost efficient, and compatible with standard semiconductor processes.